-4- Necanicum Estuary

Geologic Evidence–

Who would guess? Not far below where we stood, under the marshy grasses of the Neawanna Creek/ Necanicum River estuary, important geologic evidence was inches away. Tom Horning, an engineering geologist based in Seaside, set up his equipment to pull a tsunami-sand core sample from the estuary mud sands.

He explained that the core sample would allow him to show us changes in sedimentation that occurred as the result of a tsunami that inundated the town of Seaside in 1964.

The core sample tube was pounded into the sediments, primed, and slowly extracted from the earth as the wench worked the tube back to the surface.

Tsunami Sand Core-

The core tube was steadied on a bed of marsh grasses as Tom carefully began the process of opening it; we all watched with anticipation.

Once the sample was prepared, the sediment layers that settled on the bottom of the Neawanna/Necanicum Estuary waters were easily observed. We waited for Tom to unlock the next geologic story. Almost like a geo-“psychic” he briefly studied the lines in the soils, then he proceeded to read the strata.

The first layer was about 6 inches deep and consisted of supra tidal light brown peat and mud with roots of marsh grasses. Tom predicted that this SIX inches of collected sediments took FIFTY years to accumulate. It protected the next layer from eroding…

A fortunate geologic occurrence, to be sure, because the next SIX inch wide layer took only MINUTES (less than ten) to accumulate. How was this possible? The sands in this quickly formed stratum were derived from: the sand spit at the mouth of the Necanicum River, offshore bar, and the tidal estuary itself. They were carried by the 1964 tsunami forces unleashed by the distant earthquake up in Alaska and deposited by the in-coming energy wave as it smashed to shore over the mud bay.

Underlying the tsunami sand deposit of 1964 were peat and silt that accumulated over the decades that passed prior to the tsunami.

Why does this matter?

Tom Horning is adamant about tsunami awareness, planning, and preparation at all levels: personal, civic, and state. As a long-time resident of Seaside, Tom recounted the impact a tsunami generated by a distant earthquake had on the: environment, people, and infra-structure of his hometown. His recollection provided a valuable description of a tsunami caused by a seismic event that occurred thousands of miles away in Alaska. The details in Tom’s written account are included in National Oceanic and Atmospheric Administration (NOAA)/ Pacific Marine Environmental Laboratory tsunami education materials. It is well worth reading:

Consider this… The 1964 Good Friday Earthquake allowed time for Oregon to prepare. However, if (when) the Cascadia Subduction Zone unleashes a deep earthquake nearby along the Oregon coast, the tsunami wave energy speed of 500 mph will leave precious little time for warning and no room for real-time preparation.This fact is of abundant concern. Knowing how to react with speed and efficiency will mean the difference between survival and death. Tom Horning has been an active and vocal member of the Seaside Tsunami Advisory Group. They have worked and supported high levels of planning to identify, establish, and implement Tsunami evacuation and emergency plans.

The Cascadia Subduction Zone is an active Dynamic Earth force… it will create catastrophic events again. Although exact predictions about when this could happen are impossible to state exactly, data collected about past earthquake/tsunami scenarios suggest a strong pattern. Data sets based on carbon-dating lead geologists to conclude that subduction zone earthquakes occur in clusters. Past events have been dated and indicate that the last major seismic event happened more than 300 years ago. Calculations have shown that “magnitude 8.5 to 9.2 quakes have occurred in the Cascadia subduction zone off Oregon every 300 to 500 years for the past several thousand years.”(Horning, et al) Like it or not, we are in earthquake country…

Classroom studies with Bob, beach studies with Al, and estuary studies with Tom lead me to conclude that my knowledge about Tsunami preparedness is drastically lacking. In my post, Thoughts about the Cascadian Subduction Zone, I began to apply what I learned in this field coursework module by finding resources to expand my Tsunami Literacy. So much to think about- the links below connect with other geology stories and thoughts…

Update-

March 27, 2014

Today marked the anniversary of the Good Friday Earthquake in Southcentral Alaska that was the cause of the tsunami sands we dug up. PBS aired a segment commemorating this event on Oregon Field Guide.